(a) Field of the Invention
The present invention relates to field emission displays (FEDs) device, and more particularly, to FED devices including emitters that are realized through carbon nanotubes.
(b) Description of the Related Art
In modern FEDs a thick-layer process such as screen printing is used to form electron emission sources (i.e., emitters) in a flat configuration utilizing a carbon-based material that emits electrons at low voltage driving conditions (10-100V).
Carbon-based materials suitable for forming the emitters include graphite, diamond, diamond-like carbon, and carbon nanotubes (CNTs). Among these, carbon nanotubes appear to be very promising for use as emitters because of their extremely minute tips with a radius of curvature of approximately tens to several tens of nanometers, and because carbon nanotubes are able to emit electrons in low electric field conditions of about 1-10V/μm.
U.S. Pat. Nos. 6,062,931 and 6,097,138 disclose cold cathode field emission displays that are related to this area of FEDs using CNT technology.
In general, the FEDs employ a triode structure having cathode electrodes, an anode electrode, and gate electrodes. During manufacture of such FEDs, cathode electrodes are first formed on a substrate. Then after providing emitters on the cathode electrodes, gate electrodes are formed on the emitters.
However, with the triode structure as described above, it is difficult to satisfactorily form the emitters with holes provided in the gate electrodes and in an insulation layer formed under the gate electrodes. That is, in the process of filling the holes with an emitter material, the conductive emitter material extends between the cathode electrodes and gate electrodes to short circuit these two elements.
Further, with the conventional triode structure, when the electrons emitted from the emitters are formed into electron beams and pass over the gate electrodes (to which a positive voltage is applied) while traveling toward the phosphor layers, a diverging force of the electron beams is increased by influence of the gate electrodes such that the electron beams disperse. As a result, the electron beams land on a phosphor layer of a pixel adjacent to an intended pixel to illuminate this phosphor layer. This reduces color purity such that overall picture quality deteriorates.
To remedy this problem, there has been disclosed a configuration in which a mesh metal grid is provided between the cathode electrodes and anode electrode in an effort to realize good focusing control of the electrons emitted from the emitters. Japanese Laid-Open Patent No. 2000-268704 discloses such an FED.
In an FED having the metal grid, in addition to the advantages described, damage to the structure of the rear substrate on which the emitters are formed is prevented from arcing which results from the high voltage applied to the anode electrode. However, when electron beams are emitted from the emitters, there are electron beams that are blocked by the metal grid and do not pass through holes formed therein. These electron beams instead strike the metal grid, which decreases the utilization efficiency of the electron beams. Because the final amount of electron beams reaching the phosphors is lowered, picture brightness is reduced.
Such a problem may become worse in FEDs in which the gate electrodes are provided under the cathode electrodes and the emitters are formed on the cathode electrodes such as in U.S. Pat. No. 6,420,726 disclosed by the applicant. Since most of the emission of the electron beams occurs from the edges of the emitters, it is difficult for the electron beams to pass through the metal grid unimpaired. Accordingly, the amount of the electron beams for illuminating the phosphors is significantly reduced.
Further, in such a conventional FED in which electron emission occurs along one edge of the emitters, the electron beam emission area of the emitters is small such that there is a limit to increasing the size and density of electron beams reaching the phosphor layers. Also, with the small emission area, driving the FED for long periods in a high current range may cause damage to the emitters, thereby potentially reducing their lifespan.